wip copy on read

This is the copy-on-read feature based on both cmgitdream's (https://github.com/ceph/ceph/pull/2158) and cchengleo's (https://github.com/ceph/ceph/pull/2587) improvement. Please close https://github.com/ceph/ceph/pull/3030 and https://github.com/ceph/ceph/pull/2852 as it is easier to merge into this.
It looks like ```release``` just prevents removing the ```CopyupRequest``` from the pending list.  You also need to ensure that all delayed read/write ops have completed.  The same thing will need to be performed for the flush operations.